The Undead by Rachel Caine

After eight years of marriage, surgeon Mike Bowman and his police officer wife have snuck time into their busy schedules to celebrate their anniversary. But work is never far from Mike’s mind, which is why he finds himself in the morgue in the middle of night instead of in his wife’s arms . . .
Trying to come to terms with two young murder victims he wasn’t able to save, Mike unburdens himself to his friend and morgue attendant Adam Radburn. Good ol’ Adam, whose eyes seem bizarrely dark tonight and whose reflection in the cold storage drawers . . . doesn’t exist.
When Adam reveals his true self to Mike—and saves him from the clutches of death—Mike goes from a man trained to save lives to one who must take them to survive. And though Mike is now immune to mortal dangers, there are new threats to contend with and worse creatures who walk the Earth. One has been tracking Adam for centuries. To satisfy his bloodlust, he’s willing to destroy everyone Adam holds dear—including his new offspring.